The find confirms suspicions that some solar ejections can occur even though the surface of the sun looks tranquil.

The eruption was a coronal mass ejection (CME) – a bubble of plasma that, if energetic enough and pointed at Earth, could zap satellites, endanger astronauts, and knock out power grids (see Space storm alert: 90 seconds from catastrophe).

Warning signs

The sun ordinarily gives some warning when it is about to let loose a CME. Plasma filaments, flares, dim areas, and bright S-shaped sigmoids are often associated with the events.

However, in the past decade or so, solar physicists have measured a number of mild magnetic storms around Earth that seemed to be associated with "stealth" CME's – eruptions that occur with no clear sign of where they might have originated on the sun.

The most likely explanation was that these eruptions were simply a mild member of the CME family, capable of emerging from the Earth-facing side of the sun without dramatic warning signals.

Obscured view

But that hypothesis could not be confirmed since solar spacecraft orbiting Earth only catch a head-on view of incoming CME's, making it hard to judge the true nature of the emission. It was not entirely clear whether the eruptions were ordinary ones that somehow emerged without fanfare or something more exotic.

The twin spacecraft caught the eruption from two different vantage points as it emerged on 31 May 2008. One probe caught the outburst in profile to reveal an ordinary CME. The other captured a head-on view, finding no clear signal of activity on the sun's surface around the point where it erupted.

Shallow birth

"This proves CMEs exist that have no significant signature on the [solar] disk," says Eva Robbrecht of George Mason University in Fairfax, Virginia, who analysed STEREO's images with colleagues.

The team suspects the CME could emerge so stealthily because it did not originate deep within the sun. Instead the shallow emission gathered up material over a larger area, causing less pronounced disturbance at the star's surface.

This particular blast cast out some 3 billion tonnes of solar material, a normal amount for a CME. But the eruption was not particularly energetic, reaching a speed of about 300 kilometres per second.

If the CME had been pointed at Earth it would have created a minor geomagnetic storm by space weather standards – but strong enough to affect migratory animals and cause minor fluctuations on power grids.

Low risk

Chances are we will not have to worry about stealth CME's knocking out power grids, says Ron Moore, a solar physicist at NASA's Marshall Space Flight Center in Huntsville, Alabama.

"All the big dangerous things come from much more powerful explosions, which as far as we know are always strong enough to make some signature on the face of the sun," he says.

Even so, Moore adds, it's nice to put the question of whether stealth CME's exist to rest. "It was in some sense a mystery because we hadn't actually seen it . [Robbrecht's team has] shown that yes, indeed, that's actually what it is."

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The sun looked tranquil on 31 May 2008 (image), but it released 3 billion tonnes of material, from a region in the lower-left (Image: NASA/STEREO)